Spatial light modulator (SLM) technology has found applications in many fields, a significant one of which is that of printing. In particular, a sub-class of SLMs, known as deformable-mirror devices or digital micromirror devices (DMDs), provide significant advantages when used in printing systems.
In such systems, two types of data are typically presented. The first type of printing data is image data. Image data includes, for example, pictures of objects, such as those reproduced from photographs. With image data, the ability to print shades of gray (gray scale data) is imperative. Generally speaking, the ability to produce more shades of gray results in higher quality image printing. This ability, however, is often expensive and complicated to achieve. Thus, there is a significant need to reduce the complexity and cost of systems that can generate high quality images.
The second type of data is graphics data, such as data for text or charts. Graphics data is predominantly black and white, or other pure saturated colors. With graphics data, there is less need for gray shades. Thus, high quality graphics data can be printed so long as the resolution of the printer is high. Resolution is generally measured as the number of dots per inch that can be printed on a page. At lower resolutions, boundaries of graphics objects appear jagged. High resolution graphics require 600 dots per inch or higher. As such high resolution systems are often complex and expensive, there is an ever pressing need to reduce their cost and complexity.
Existing electro photo-graphic printer technologies make use of an organic photoconductive (OPC) drum. Depending on the type of photoconductor used, the drum is either charged or discharged to attract toner, with the charging or discharging accomplished by reflecting light onto the drum from a DMD array. Ideally, the amount of toner that clings to any point on the drum would be a function of the level of charge (or discharge) on that point. In this ideal case, gray scaling could be done simply by adjusting the charge or discharge of each point so as to control the amount of toner on any point, and thus the gray scale printed. However, with existing technologies, toner clings to the drum in such a manner that typically about four to thirty-two levels of gray can be achieved by controlling the charge on a particular point on the drum.
Therefore, gray scales of the kind required for high resolution imaging can be produced only by taking advantage of these relatively few levels of gray scale and the ability of the human eye to integrate over an area. For example, a mid-level gray dot will perceived if smaller dots of lighter and darker than mid-level gray are printed next to each other. For example, if two lighter gray dots of 1/600 of an inch square and two darker gray dots of 1/600 of an inch square are printed next to each other, the eye will integrate the four dots and perceive a mid-gray of the size of about 1/300 of an inch square. With DMD technology, this may be accomplished by using a high number of small mirrors, a technique that is expensive and complicated.
With OPC drum printing as described above, a page is printed by writing data to the drum array by array. The direction the drum turns is known as the process direction. As the drum rotates, overlapping arrays of data are superposed on the drum as light exposure is accumulated on the drum by integration of several DMD array exposures.
Two types of light modulation can be achieved by using DMD technology: intensity modulation and spatial/area modulation. Techniques have been presented for printing gray scales by the use of intensity modulation, and also area modulation in the process direction. See for example, copending U.S. patent application Ser. No. 08/038,398, filed Mar. 29, 1993, entitled "Process and Architecture for Digital Micromirror Printer," TI-17632, assigned to Texas Instruments Incorporated. Area modulation was achieved by overlapping DMD exposures on the drum by non-integer displacements (an integer displacement by one pixel re-aligns the current exposure with the previous exposure). Such techniques, and systems do not allow, however, for the generation of gray scales by taking advantage of area modulation in the cross-process direction.
Therefore, a need has arisen for a method and apparatus that allows for the generation of gray scales by achieving spatial modulation in both the process and cross-process direction. Furthermore, a need has arisen for a method and apparatus for more accurate addressing of pixels to allow for high quality graphics printing without significant cost or complexity.